PNIVEBSITY  OF  CALIFORNIA  f PBLICATIOM8 

COLLEGE  OF  AGRICULTURE 

AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY,  CALIFORNIA 


SUN-DRYING  AND  DEHYDRATION 
OF  WALNUTS 


BY 

L.  D.  BATCHELOR  and  A.  W.  CHRISTIE 

ASSISTED  BY 

E.  H.  GUTHIER  and  R.  G.  LaRUE 


BULLETIN  No.  376 

March,  1924 


UNIVERSITY  OF  CALIFORNIA  PRINTING  OFFICE 

BERKELEY,  CALIFORNIA 

1924 


Digitized  by  the  Internet  Archive 

in  2012  with  funding  from 

University  of  California,  Davis  Libraries 


http://www.archive.org/details/sundryingdehydra376batc 


SUN-DRYING  AND  DEHYDRATION  OF  WALNUTS* 

BY 

L.  D.  BATCHELOR  and  A.  W.  CHRISTIEt 

ASSISTED    BY 

E.  H.  GUTHIER  and  R,  G.  LaRUE 


CONTENTS  page 

Methods  of  Sun-Drying 4 

Cost  of  Sun-Drying 6 

The  Use  of  Unheated  Drying  Houses 7 

Early  History  of  Walnut  Dehydration 8 

Recent  Interest  in  Artificially  Drying  Walnuts 8 

Extent  of  Dehydration  During  1923 10 

Advantages  of  Dehydration 10 

Weather  Protection 10 

Theft  Protection 10 

Drying  Efficiency 10 

Labor  Efficiency 11 

Packing  House  Efficiency 11 

Marketing  Efficiency 11 

Principles  of  Dehydration 11 

Temperature  Effects 13 

Critical  Temperature  for  Quality 13 

Splitting  of  Nuts  during  Curing 15 

Relation  of  Temperature  to  Drying  Time 15 

Starting  Dehydration  at  Low  Temperature 15 

Intermittent  Operation  of  Dehydraters 16 

Relation  of  Humidity  to  Dehydration.... 17 

Relation  of  Air  Flow  to  Dehydration.. 17 

Relation  of  Depth  of  Walnuts  to  Air  Flow 18 

Yields  and  Storage  of  Dehydrated  Nuts 20 

Types  of  Walnut  Dehydraters 21 

Natural  Draft  Dryers 21 

Air  Blast  Dehydraters 21 

Bin  Dehydraters 22 

Tray  Dehydraters 24 

Mechanical  Movement  of  Nuts  During  Drying 25 

Costs  of  Dehydration 25 

*  Paper  No.  117,  University  of  California,  Graduate  School  of  Tropical  Agri- 
culture and  Citrus  Experiment  Station,  in  cooperation  with  Division  of  Viticulture 
and  Fruit  Products. 

t  The  authors  wish  to  acknowledge  the  assistance  rendered  by  the  California 

Walnut  Growers'  Association  in  donating  Mr.  LaRue's  services  as  well  as 
appropriating  a  fund  to  further  this  work. 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


SUN-DRYING  AND  DEHYDRATION  OF  WALNUTS 


Walnuts  must  be  dried  or  "cured"  before  they  can  be  marketed. 
This  is  generally  done  by  exposing  them  to  the  sun  for  several  days. 
The  seasonal  drying  operations  usually  extend  over  six  or  eight  weeks, 
and  in  cool  or  moist  weather  may  be  very  slow  and  imperfect. 

Improper  curing  may  reduce  good  nuts  to ' '  culls ' '  or ' '  near-grades ' ' 
and  depreciate  their  value  15  to  50  per  cent  and  delay  their  delivery 
at  the  packing  house  until  the  association's  "second  pool,"  which  sells 
at  a  discount  of  10  or  15  per  cent  below  the  ' '  first  pool. ' '  Non-associa- 
tion growers  are  not  directly  affected  by  the  closing  of  pools  but  it  is 
the  usual  practice  of  commercial  buyers  to  accept  late  deliveries  only 
at  a  discount. 

Losses  of  this  kind  caused  by  the  periodical  occurrence  of  damaging 
weather  in  some  of  the  walnut  growing  districts  account  for  the  present 
interest  in  walnut  dehydration. 

As  shown  in  detail  on  page  10,  the  advantages  of  dehydration  in 
promoting  fine  quality  and  early  marketing  because  of  rapid  curing, 
will  be  of  great  value  in  helping  to  profitably  market  the  rapidly 
increasing  crops  of  California  walnuts  in  competition  with  foreign 
walnuts. 

METHODS    OF    SUN-DRYING 

Walnut  drying,  commonly  referred  to  as  curing,  is  for  the  purpose 
of  evaporating  the  excess  moisture  in  the  nuts  which,  if  not  removed, 
would  lead  to  spoilage  and  a  lack  of  stability  in  weight.  The  method 
as  practiced  by  the  pioneers  of  the  industry  was  described  by  Joseph 
Sexton*  in  1888 : 

The  walnuts  are  picked  up  and  put  in  sacks  and  barrels,  so  as  to  be  easily 
handled,  and  hauled  to  a  sunny  place  to  dry,  and  should  be  placed  on  ele- 
vated platforms  made  of  narrow  boards,  with  spaces  of  one-fourth  of  an 
inch  between  each  board.  The  platform  should  be  about  8  feet  wide  and  40 
feet  long,  or  as  long  as  two  men  can  handle  a  canvas  to  cover  the  beds,  which 
should  be  done  every  night  the  dew  falls.  The  nuts  should  be  stirred  in  these 
beds  once  or  twice  each  day,  and  with  favorable  weather  they  will  dry  sufficiently 
in  three  days,  and  are  ready  for  market. 


Paper  read  before  Ninth  Fruit-Growers'  Convention,  1888. 


BULLETIN   376]  SUN-DRYING  AND   DEHYDRATION   OF   WALNUTS  5 

Most  of  the  walnuts  grown  in  California  are  now  dried  on  trays 
spread  in  the  sunshine  or  in  thoroughly  ventilated  drying-houses.  The 
practice  is  as  follows :  The  nuts  are  spread  on  shallow  slat-bottom  trays 
and  exposed  to  sun  and  wind.  If  the  air  is  especially  dry  and  hot,  the 
trays  should  be  spread  out  in  the  morning,  and  when  the  nuts  are  well 
warmed  they  should  be  thoroughly  stirred  and  the  trays  piled  up.  The 
slower  drying  which  goes  on  while  the  trays  are  in  piles  avoids  the 
splitting  of  poorly  sealed  nuts  and  their  rejection  as  culls.  Figure  1 
shows  a  yard  of  trays  being  piled  up  after  having  been  thoroughly 
warmed  by  the  morning  sun.  The  trays  should  be  so  piled  as  to  allow 
ventilation  between  them. 


Fig.  1. — Trays  ready  to  pile  up  after  having  been  thoroughly  warmed 
by   morning   sunshine. 


The  walnuts  should  not  be  exposed  to  showers,  fogs,  or  even  heavy 
dews  during  the  curing  process.  The  alternate  wetting  and  drying 
of  the  shells  which  occur  when  the  nuts  are  exposed  to  dew  at  night 
and  to  sunshine  during  the  day,  cause  many  of  the  nuts  to  crack  at  the 
apex.  During  the  season  of  1923,  two  trays  of  nuts  were  cured  under 
identical  conditions  except  that  one  was  covered  from  sun-down  to 
sun-rise  while  the  other  was  exposed  to  the  dew  and  fog.  On  the 
uncovered  tray  64  per  cent  of  the  nuts  cracked,  on  the  covered  only 
4  per  cent. 

The  trays  most  commonly  used  are  3  by  6  feet,  and  6  inches  deep. 
Extending  the  sides  of  the  trays  12  inches  beyond  the  ends  makes 
suitable  handles.     The  bottoms  are  made  of  laths  spaced  one-half  an 


b  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

inch  apart.  The  present  price  for  such  trays  is  approximately  $2.00 
each.  Supports  are  needed  for  the  trays  to  allow  air  circulation  under 
and  through  them  while  the  nuts  are  drying.  Waterproof  covers  are 
used  for  the  stacked  trays  to  keep  the  nuts  dry  during  rain  or  foggy 
weather. 

COST   OF   SUN-DRYING 

The  cost  of  curing  walnuts  on  trays  in  the  sun  was  made  the  subject 
of  a  survey  during  the  harvest  period  of  1923.  Twenty-five  walnut 
growers  were  interviewed  in  the  several  walnut  growing  districts  from 
Irvine  to  Goleta.  The  average  cost  per  ton  amounted  to  $5.48,  as  shown 
in  table  1.  This  included  the  overhead  charges  of  depreciation  and 
interest  on  capital  invested  in  trays  and  the  labor  costs  in  the  drying 
yard  from  the  time  the  nuts  were  received  from  the  grove  until  they 
were  sacked  ready  for  delivery  to  the  packing  house.  The  contract 
price  for  performing  this  service  in  the  districts  visited  averaged  $7.35 
a  ton.  According  to  figures  gathered  in  1920  by  the  Walnut  Protective 
League,*  the  cost  of  curing  walnuts  amounted  to  approximately  $10.00 
a  ton. 

TABLE   1 

Cost  of  Sun-drying  Walnuts* 


Tray  invesment 
per  ton 

Labor 
per  ton 

Overhead  charge 
per  ton 

Number 

trays 
required 

Total 
cost 

Man 
Hours 

Cost  at 
40  cents 
per  hour 

Interest 

at 
6  per  cent 

Depre- 
ciation 

at  5 
per  cent 

Total  cost 
per  ton 

Maximum 

Minimum 

Average 

12.00 

2.66 

7.78 

$24.00 

5.32 

15.56 

35.42 

0.82 
9.45 

$14.17 

.33 

3.77 

$1.44 
.32 
.93 

$1.20 

.27 
.78 

$16.81 

.92 

5.48 

*  Data  from  25  growers  producing  from  7  to  135  tons,  using  standard  3x6 
trays. 

The  survey  summarized  in  table  1  indicates  that  the  method  of  sun- 
drying  on  trays  requires  8  trays  per  ton  of  nuts  cured  during  the 
season.  The  average  production  of  the  groves  surveyed  was  1200 
pounds  per  acre.  With  an  average  production  of  1000  pounds  per 
acre  a  10-acre  grove  would  require  40  trays  to  properly  cure  the  crop. 
The  investment  in  trays  alone  would  approximate  $80.00  per  10-acre 
unit.  In  addition  to  this  most  growers  have  sheds  in  which  to  house 
the  trays  when  not  in  use  and  for  the  temporary  storage  of  cured  nuts. 


*  American  Walnut  Industry,  by  Walnut  Protective  League,  Jan.  1921,  p.  23. 


Bulletin  376] 


SUN-DRYING   AND   DEHYDRATION    OF   WALNUTS 


The  greatest  single  charge  is  for  labor.  According  to  the  growers 
interviewed,  it  requires  an  average  of  4  man  hours  per  day  to  care  for 
100  trays.  Stated  on  a  tonnage  basis  the  average  labor  cost  equalled 
$3.77  per  ton.  It  is  often  difficult  to  obtain  the  necessary  labor  as  the 
demand  for  labor  during  the  walnut  harvest  season  usually  exceeds 
the  supply. 

Despite  certain  disadvantages,  sun-drying  has  always  been  used  in 
the  curing  of  most  of  the  crop.  For  small  orchards  it  will  likely 
continue  to  be  the  most  practicable  method. 


Fig. 


-Sidwell  drying 


house,  outer  walls  made  of  slats  to  give  good 
ventilation. 


THE    USE    OF    UNHEATED    DRYING    HOUSES 

In  curing  large  crops  it  will  save  labor  to  use  a  drying-house  such 
as  the  Sidwell  dryer  illustrated  in  figure  2.  The  drying  houses  are 
usually  built  with  outer  walls  of  slats  to  give  good  ventilation.  The 
drying  trays  or  floors  are  arranged  one  above  another,  8  to  10  in 
number.  The  nuts  are  elevated  by  conveyors  to  the  top  floor  on  which 
they  are  spread  in  a  layer  about  6  inches  deep.  As  drying  progresses 
the  nuts  are  dumped  mechanically  from  one  floor  to  the  floor  below  by 
means  of  pivoted  sections  in  the  floors.  The  frequency  with  which  the 
nuts  are  dumped  and  consequently  the  number  of  days  required  for 
drying  will  depend  on  the  moisture  content  of  the  nuts  and  weather 
conditions,  usually  5  to  7  days  in  favorable  weather. 


8  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

In  hot  dry  weather  the  nuts  may  pass  through  the  drying  house  in 
4  to  5  days.  To  realize  their  maximum  efficiency  such  drying  houses 
should  be  exposed  to  the  full  effect  of  wind  and  sun  without  inter- 
ference from  adjacent  buildings  or  trees. 

When  the  nuts  on  the  lowest  floor  are  cured  they  are  dumped  into 
hoppers  from  which  a  conveyor  transfers  them  to  one  end  of  the  drying 
house  for  sacking.  While  the  nuts  pass  along  this  conveyor,  there  is 
an  excellent  opportunity  for  an  initial  culling  before  delivery  to  the 
packing  house. 

The  only  advantages  of  such  unheated  drying  houses  are  that  the 
nuts  are  protected  from  rains,  and  that  the  labor  cost  per  ton  of  nuts 
cured  is  less  than  for  tray  drying,  but  this  saving  is  cancelled  by  the 
greater  overhead  charges  on  the  equipment.  The  saving  in  labor  when 
the  walnut  and  other  farm  crop  demands  are  at  a  peak  may  make  the 
use  of  a  drying  house  more  practicable  than  tray  drying  even  though 
it  may  be  more  costly. 

The  advantages  of  modern  heated  dryers  for  walnuts  are  obtainable 
at  no  greater  cost  per  ton  than  for  unheated  drying  houses.  (See 
Advantages  of  Dehydration,  p.  10.) 

EARLY    HISTORY   OF   WALNUT    DEHYDRATION 

The  curing  of  walnuts  by  artificial  heat  apparently  began  in  the 
late  eighties.  Hon.  Russel  Heath*  referred  to  the  methods  used  in 
artificially  drying  walnuts  at  that  time,  as  follows : 

In  handling  the  nuts,  I  cure  in  dry-houses  by  artificial  heat,  heating  sufficient 
to  evaporate  the  water  and  set  the  oil  of  the  nut.  When  this  is  done,  the  nuts 
will  keep  sweet  for  an  indefinite  time. 

Apparently  the  early  methods  of  artificially  drying  walnuts  were 
not  entirely  practicable,  at  least  the  industry  was  not  converted  to  this 
method  of  curing. 

RECENT   INTEREST   IN   ARTIFICIALLY   DRYING   WALNUTS 

Aside  from  the  artificial  drying  of  walnuts  by  a  few  of  the  early 
pioneers  in  the  industry,  there  have  been  only  a  few  cases  of  walnuts 
being  dried  by  this  method  until  recently.  Among  the  advocates  of 
this  method  of  drying  walnuts,  the  following  may  be  mentioned : 

The  Corona  del  Mar  Ranch  at  Goleta  has  dried  walnuts  in  an  old 
prune  dryer  for  the  past  15  to  20  years.     The  nuts  are  held  in  trays 


*  Essay  read  by  Hon.  Eussel  Heath  before  the  Eleventh  State  Fruit  Growers : 
Convention  1889. 


BULLETIN   376]  SUN-DRYING  AND  DEHYDRATION   OF   WALNUTS  9 

over  a  hot  air  flue,  which  is  heated  by  a  wood  fire  in  a  brick  furnace. 
The  temperature  maintained  is  based  only  on  judgment  and  is  regu- 
lated by  the  draft  of  the  furnace.  The  nuts  are  received  in  the  dryer 
after  passing  through  the  bleaching  solution,  and  are  then  warmed  for 
a  period  of  10  to  12  hours. 

In  1918  Mr.  C.  I.  Crane  of  Santa  Paula  began  the  practice  of  drying 
walnuts  with  heat  from  a  furnace.  The  nuts  were  placed  in  a  wooden 
slat  bottom  bin  with  the  heat  entering  at  the  bottom.  In  1919  the 
dryer  was  completely  burned.  In  1922  a  new  dryer  was  installed  using 
steam  heating  coils  underneath  a  wooden  slat  bottom  bin.  Tempera- 
tures of  from  100°  to  118°  F.  have  been  maintained,  and  the  nuts  dried 
in  from  12  to  48  hours  according  to  their  moisture  content. 

"During  the  summer  of  1920,  the  San  Joaquin  Fruit  Company* 
installed  a  small  crude-oil  burner  and  a  blower  to  force  the  heated  air 
to  the  drying  shed.  A  bin  was  built  which  would  hold  ten  tons  of 
walnuts,  or  a  day's  pick,  and  during  bad  weather  the  walnuts  were 
put  in  this  bin  and  hot  air  at  about  85°  to  90°  F.,  was  forced  in  at  the 
bottom  for  about  18  hours,  when  the  bin  was  emptied  and  the  process 
repeated  with  green  nuts.  In  this  manner  the  outside  moisture  was 
driven  off,  and  the  curing  process  in  the  trays  was  made  shorter. ' ' 

From  this  bin  dryer  the  nuts  were  conducted  on  a  belt  to  the  floors 
in  the  drying  house. 

During  the  summer  of  1920,  Mr.  F.  T.  Mahoney  built  a  walnut 
dryer  for  Mr.  E.  C.  Kimble  of  Saticoy.  The  nuts  were  poured  into  a 
wooden  bin  with  a  slanting,  slat  bottom;  hot  air  was  forced  into  a 
chamber  below  this  bin,  and  the  draft  of  hot  air  passed  through  the 
nuts.    In  1920  there  were  26  tons  of  nuts  dried  in  this  bin  dryer. 

During  the  summer  of  1921  the  dryer  on  the  Kimble  ranch  was 
rebuilt,  the  bin  being  divided  into  four  compartments.  The  fan  for 
delivering  heated  air  to  the  bins  was  changed  from  a  high  speed  blower 
to  a  slower  speed  Multivane  fan  which  delivered  a  greater  volume  of 
air  per  horsepower  consumed. 

Conveying  machinery  was  added  to  the  Kimble  dryer  before  the 
harvest  season  of  1922.  Three  similar  dryers  were  installed  on  other 
ranches  the  same  season.  The  addition  of  the  conveying  machinery  for 
elevating  the  nuts  from  a  hopper  into  the  bins  was  a  labor-saving 
device  of  pronounced  value.  The  interest  in  this  type  of  bin  dryer 
increased  in  1923,  when  ten  additional  dryers  of  this  type  were  con- 
structed in  Ventura  County. 


*  From  address  by  C.  V.  Newman,  on  Walnut  Curing,  read  before  Fourth 
Annual  Walnut  Institute,  Jan.  1922. 


10  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  general  interest  in  curing  walnuts  by  artificial  heat  was  greatly 
stimulated  during  the  spring  of  1922,  by  an  address  by  Mr.  Carl 
Newman  before  the  Fourth  Annual  Walnut  Institute,  held  at  Santa 
Ana.  Mr.  Newman  pointed  out  the  fact  that  methods  of  curing  wal- 
nuts had  received  very  little  attention  or  modification  since  the  early 
development  of  the  walnut  industry.  As  a  result  of  the  stimulated 
interest  in  this  subject,  the  California  Walnut  Growers'  Association 
appropriated  a  fund  of  $1000  to  be  used  in  studying  walnut  dehydra- 
tion. This  fund  has  been  expended  by  the  field  department  of  the 
Association,  in  cooperation  with  the  Agricultural  Experiment  Station 
of  the  University  of  California.  The  experiments  herein  described 
may  be  considered  a  report  of  the  expenditure  of  this  fund,  for  without 
this  assistance  in  financing  the  investigation,  the  University  would  have 
been  obliged  to  greatly  curtail  its  present  activity  on  this  problem. 

EXTENT    OF    DEHYDRATION    DURING     1923 

In  1923  there  were  22  walnut  dehydraters  of  various  types  in 
operation.  The  average  crop  dried  per  dehydrater  was  72.5  tons,  mak- 
ing a  total  amount  of  1593  tons.  This  quantity  amounts  in  general 
terms  to  6.5  per  cent  of  the  total  crop  of  walnuts  in  California.  Of  the 
22  dehydraters  in  operation  in  1923,  13  were  operated  for  the  first 
time  that  season. 

ADVANTAGES  OF  DEHYDRATION 

Careful  observation  of  the  operation  of  all  types  of  walnut  dehy- 
draters during  the  past  two  seasons  has  led  to  the  following  conclusions 
regarding  the  advantages  of  dehydration  as  compared  with  sun-drying 
in  curing  walnuts. 

1.  Weather  Protection. — Dehydraters  may  be  constantly  operated 
and  thus  continue  the  drying  process  day  and  night  in  any  kind  of 
weather.  Splitting  of  nuts  by  dry,  hot  winds  or  delayed  drying 
because  of  fogs  or  rains  are  both  avoided,  with  consequent  elimination 
of  worry,  extra  labor,  depreciation  in  the  quality  of  the  nuts,  and  a 
general  financial  loss  to  the  grower  during  unfavorable  drying  weather. 

2.  Theft  Protection. — All  harvested  nuts  are  protected  from  theft 
by  being  dried  and  stored  within  the  dehydrater  building  until 
delivered  to  the  packing  house. 

3.  Drying  Efficiency. — Accurate  control  of  the  drying  air  results 
in  the  most  uniform,  thorough,  and  rapid  drying  consistent  with  the 
best  quality. 


Bulletin   376]  SUN-DRYING  AND  DEHYDRATION   OF   WALNUTS  11 

4.  Labor  Efficiency. — Dehydraters  require  considerably  less  labor 
per  ton  of  nuts  dried.  This  is  an  important  consideration  in  the  fall 
of  the  year  when  the  harvesting  of  other  crops  is  making  demands  on 
the  labor  supply  in  competition  with  walnuts. 

5.  Packing  House  Efficiency. — Dehydraters  afford  a  steady  supply 
of  cured  nuts,  enabling-  packing  houses  to  more  nearly  operate  at  their 
maximum  efficiency  throughout  the  season. 

The  effect  of  some  fifteen  dehydraters  operating  in  the  vicinity  of 
the  Saticoy  packing  house  in  1923  on  the  daily  receipts  of  that  house 
as  compared  with  1922  is  illustrated  in  figure  3.  Even  the  influence 
of  these  few  dehydraters  did  much  to  equalize  the  daily  delivery  of  nuts 
to  the  packing  house  with  consequent  reduction  in  the  cost  of  packing. 

The  leveling  effect  upon  the  packing  load  of  the  Saticoy  house 
during  the  two  seasons  may  be  summarized  as  follows:  The  daily 
packing  capacity  of  the  house  is  110  tons;  the  maximum  storage  capac- 
ity is  200  tons.  During  1922  the  house  used  extra  crews  to  operate  the 
house  7  Sundays  and  16  nights.  During  1923  the  house  operated  only 
1  Sunday  and  6  nights. 

6.  Marketing  Efficiency. — Dehydration  gains  several  days  over  sun- 
drying;  thereby  facilitating  early  marketing  which  is  growing  in 
importance  as  the  tonnage  of  California  walnuts  increases.  The  peak 
of  demand  for  walnuts  occurs  before  the  period  of  the  peak  of  delivery, 
therefore,  anything  which  will  tend  to  speed  up  the  wholesale  delivery, 
will  stimulate  the  market.  With  ordinary  harvesting  methods  plus  the 
use  of  a  dehydrater  a  grower  can  fe  1  certain  that  his  entire  crop  will 
reach  the  packing  house  before  the  "first  pool"  closes.  The  earlier 
packing  house  deliveries  illustrated  in  figure  3  were  largely  the  result 
of  dehydration. 

PRINCIPLES    OF    DEHYDRATION 

Dehydration  may  be  defined  as  the  evaporation  of  excess  moisture 
without  injury  to  the  products  by  carefully  controlled  currents  of 
artificially  heated  air.  This  modern  method  of  drying  has  recently 
received  much  study  and  development  in  connection  with  the  drying 
of  fruits  in  California.  The  fundamental  engineering  principles  under- 
lying the  construction  and  operation  of  dehydraters  for  walnuts  are 
very  similar  to  those  already  published  for  fruits.*  It  was  necessary, 
however,  to  study  the  behavior  of  walnuts  during  dehydration  in  order 
to  determine  what  types  of  equipment  and  methods  of  operation 
resulted  in  the  maximum  efficiency  in  drying  compatible  with  fine 
quality  and  economical  operation. 

*  The  Construction  of  Farm  Dehydraters  in  California,  by  A.  W.  Christie  and 
G.  B.  Ridley,  Journal  of  the  American  Society  of  Heating  and  Ventilating  Engi- 
neers, Vol.  29,  No.  9.    December,  1923. 


12 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


The  time  required  to  evaporate  the  excess  moisture  in  walnuts  is 
dependent  on  the  following  factors : 

1.  The  maximum  temperature   at  which  walnuts  can  be   safely 
dehydrated. 

2.  The  per  cent  of  excess  moisture  in  the  nuts  when  harvested. 

3.  The  volume  of  heated  air  reaching  the  nuts. 

4.  The  relative  humidity  of  the  air. 


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DAJLY   PACK/NG     CAPAC/TY 

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60 

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/923 

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DA/LY  PACff/NG    CAPAC/TY 

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60 

60 

40 

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Fig.  3. — Comparative  daily  delivery  of  nuts  to  Satieoy  packing  house. 


Bulletin  376] 


SUN-DRYING  AND   DEHYDRATION    OF   WALNUTS 


13 


The  following  paragraphs  set  forth  briefly  the  results  of  numerous 
tests  on  the  dehydration  of  walnuts  and  the  bearing  of  this  informa- 
tion on  the  construction  and  operation  of  walnut  dehydraters : 

TEMPERATUEE  EFFECTS 
The  evaporation  of  water  requires  the  expenditure  of  a  definite 
amount  of  heat  (approximately  1000  British  Thermal  Units  per  pound 
of  water  evaporated).  In  the  case  of  walnut  dehydraters  this  heat  is 
most  conveniently  and  economically  generated  in  oil-burning  furnaces. 
It  is  obvious  that  the  higher  the  temperature  of  the  air  conveying  this 
heat  from  the  furnace  to  the  nuts,  the  more  rapid  will  be  the  evapora- 
tion of  moisture  from  the  nuts. 


TABLE  2 
Relation  of  Temperature  op  Dehydration  to  Quality  of  Walnuts 


Per  cent 

Drying 
tempera- 
ture 

Percentage 

showing 

oil  on 

kernel 

Eating  quality 

water 

when 

harvested 

After 
drying 

After  4 
months* 

32 

100°F. 

76% 

normal 

normal 

37 

100°F. 

28% 

normal 

normal 

46 

105°F. 

15% 

normal 

normal 

38 

110°F. 

30% 

normal 

injured  (stale  and  bitter) 

30 

110°F. 

44% 

normal 

18 

110°F. 

20% 

normal 

normal. 

22 

110°F. 

56% 

normal 

slightly  injured 

17 

115°F. 

34% 

slightly  injured 

36 

120°F. 

60% 

normal 

injured  (flat,  slightly- 
rancid) 

29 

130°F. 

80% 

pleasant  but 
cooked  flavor 

rancid  (not  edible) 

35 

140°F. 

100% 

unpleasant  oily 
flavor 

rancid  (not  edible) 

*  Held  in  common  storage. 

Critical  Temperature  for  Quality. — Most  food  products  have  a 
"critical"  temperature  beyond  which  they  cannot  be  heated  without 
danger  of  injury  to  their  quality.  The  results  of  a  series  of  carefully 
controlled  temperature  tests  are  shown  in  table  2,  from  which  it  can  be 
seen  that  the  "critical"  temperature  for  walnut  dehydration  is  in  the 
neighborhood  of  110°  F.  It  was  also  evident  that  the  higher  the  per 
cent  of  moisture  in  the  nuts  when  harvested,  the  lower  the  ' '  critical ' ' 
temperature.  The  dehydrated  nuts  were  always  compared  with  sun- 
dried  nuts  which  came  from  the  same  respective  original  lots.    It  may 


14 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


be  concluded,  therefore,  that  in  no  case  should  walnut  dehydraters  be 
operated  above  110°  F.,  and  in  the  case  of  very  moist  nuts  from  early 
harvesting1,  it  is  imperative  not  to  exceed  105°  F.  Walnuts  dehydrated 
in  1922  were  still  in  first-class  marketable  condition  at  the  end  of  18 
months. 

TABLE  3 

Comparative  Drying  Data  on  Sun-drying  and  Dehydration  of  Walnuts 
(Orange  County,  1923) 


Date  of 
harvest 

Method  of 
drying 

Per  cent 

water  in 

nuts  before 

curing 

Per  cent 

of  weight 

lost  in 

drying 

Per  cent 

of  water 

in  cured 

nuts 

Per  cent 
of  nuts 

unsealed 
after 
curing 

Drying 
time, 
hours 

Drying 
tempera- 
ture 

Sept.  15 

Sundried 

46.25 

32.32 

8.87 

2.25 

224 

65° 

Sept.  15 

Dehydrated 

46.25 

36.19 

5.11 

0.00 

46 

105° 

Sept.  17 

Sundried 

37.70 

31.56 

7.25 

4.00 

240 

58° 

Sept.  17 

Dehydrated 

37.70 

33.57 

5.48 

0.00 

36 

110° 

Sept.  19 

Sundried 

36.00 

25.78 

7.83 

8.00 

200 

64° 

Sept.  19 

Dehydrated 

36.00 

30.57 

5.66 

0.15 

24 

120° 

Sept.  20 

Sundried 

28.76 

27.14 

6.94 

5.50 

195 

64° 

Sept.  20 

Dehydrated 

28.76 

27.72 

7.45 

0.27 

18 

130° 

Sept.  22 

Sundried 

35.11 

33.21 

6.79 

4.00 

170 

65° 

Sept.  22 

Dehydrated 

35.11 

27.64 

6.91 

0.04 

14 

140° 

Sept.  24 

Sundried 

36.60 

30.30 

8.04 

2.00 

198 

66° 

Sept.  24 

Dehydrated 

36.60 

30.05 

9.29 

0.08 

30 

100° 

Sept.  25 

Sundried 

33.18 

29.85 

8.02 

2.00 

222 

67° 

Sept.  25 

Dehydrated 

33.18 

27.90 

8.49 

0.07 

42 

90° 

Sept.  28 

Sundried 

31.55 

27.14 

3.67 

9.00 

198 

72° 

Sept.  28 

Dehydrated 

31.55 

25.30 

7.00 

0.08 

32 

100° 

Sept.  30 

Sundried 

29.82 

20.53 

3.94 

6.50 

122 

67° 

Sept.  30 

Dehydrated 

29.82 

20.59 

4A4: 

0.00 

46* 

110° 

Oct.      2 

Sundried 

18.45 

17.50 

3.98 

6.00 

184 

67° 

Oct.     2 

Dehydrated 

18.45 

14.24 

4.74 

0.00 

32f 

110° 

Oct.      8 

Sundried 

21.64 

16.03 

3.77 

11.00 

120 

68° 

Oct.     8 

Dehydrated 

21.64 

16.96 

5.52 

0.28 

28 

110° 

Oct.    10 

Sundried 

16 .  72 

9.69 

4-04 

13.00 

96 

71° 

Oct.    10 

Dehydrated 

16.72 

14.85 

2.47 

0.86 

20 

115° 

Averages  -1 

Sundried 

30.89 

25.09 

6.26 

6.10 

181. 

66° 

Dehydrated 

30.89 

25.46 

6.09 

0.15 

31. 

112° 

*  Includes  10-hour  shut-down  after  12  hours  of  drying.     Fan  Off. 
t  Includes  10-hour  shut-down  after  12  hours  of  drying.     Fan  On. 

Because  of  the  danger  from  higher  temperatures,  it  is  important 
that  all  walnut  dehydraters  be  adequately  equipped  with  accurate 
thermometers.  The  temperature  of  dehydraters  heated  by  oil,  gas, 
steam  or  electricity  can  be  automatically  controlled  by  thermostats 
which  eliminate  the  danger  of  over-heating  as  well  as  the  necessity  for 
the  frequent  temperature  observations  in  manually  operated  plants. 


BULLETIN   376]  SUN-DRYING  AND  DEHYDRATION   OF  WALNUTS  15 

Splitting  of  Nuts  during  Curing. — Another  valuable  observation 
associated  with  quality  is  noted  in  the  sixth  column  of  table  3.  The 
average  per  cent  of  nuts  split  during-  dehydration  was  only  0.15 
per  cent  as  compared  with  6.1  per  cent  during  sun-drying  of  similar 
lots.  In  no  case  did  the  per  cent  of  split  dehydrated  nuts  reach  1 
per  cent,  while  in  sun-drying  it  rose  as  high  as  13  per  cent.  This  is  a 
distinct  advantage  in  favor  of  dehydration. 

Relation  of  Temperature  to  Drying  Time. — The  comparative  drying 
times  of  sun-drying  and  of  dehydrating  nuts  can  be  seen  in  the  seventh 
column  of  table  3.  Sun-drying  required  from  4  to  10  days,  averaging 
7!/2  days,  while  dehydration  required  from  14  to  46  hours,  averaging 
31  hours.  The  greater  speed  of  dehydration  (about  six  times  as  fast) 
is  due,  first,  to  the  higher  temperature  obtainable,  and,  second,  to  the 
maintenance  of  this  temperature  throughout  the  drying  period.  Most 
of  the  daily  evaporation  of  water  in  sun-drying  occurs  during  a  few 
hours  in  the  middle  of  the  day,  while  dehydration  is  continuous  until 
completed.  Cool,  moist  nights  alternating  with  warm,  dry  days  not 
only  delay  drying  but  cause  the  splitting  of  nuts  from  alternate 
expansion  and  contraction. 

It  is  difficult  to  give  an  average  drying  time  for  walnuts  at  different 
temperatures  because,  although  the  conditions  of  dehydration  be  under 
exact  control,  the  initial  percentages  of  moisture  in  different  lots  of 
nuts  vary  greatly.  However,  the  following  averages  obtained  from  all 
experiments  made  in  1922  and  1923  conclusively  show  the  advantage 
of  operating  a  walnut  dehydrater  at  the  maximum  safe  temperature. 

Hours  required 
to  evaporate 
Temperature  1  per  cent 

of  moisture 

90°  F 2.56 

100°  F '. 2.17 

110°  F •. 1.51 

The  use  of  temperatures  below  the  maximum  safe  temperature 
merely  results  in  lengthening  the  time  of  drying  with  consequent 
decrease  in  the  capacity  of  the  dehydrater  and  increase  in  the  cost  of 
drying. 

Starting  Dehydration  at  Low  Temperatures. — No  valid  objection 
was  found  to  beginning  the  dehydration  of  walnuts  at  the  maximum 
temperature  to  be  used.  Slow  "warming  up"  of  the  nuts  merely 
increased  the  time  and  cost  of  drying. 


16 


UNIVERSITY    OF   CALIFORNIA — EXPERIMENT    STATION 


INTEEMITTENT    OPERATION   OF    DEHYDRATERS 

A  number  of  dehydrater  operators  practice  intermittent  operation, 
the  furnace  and  fan  being  shut  off  for  about  10  hours  during  every 
night.  It  was  thought  that  the  nuts  would  continue  drying  during  this 
period.  This  practice  was  made  the  object  of  an  exact  study,  the 
results  of  which  are  shown  in  figure  4.  It  is  evident  that  as  soon  as 
the  flow  of  heated  air  ceases,  there  is  little  or  no  further  drying  of  the 
nuts  until  the  dehydrater  is  started  again.  In  certain  instances  there 
may  be  other  factors,  such  as  labor  supply,  which  justify  intermittent 
operation,  but  from  a  standpoint  of  drying  efficiency  it  merely  reduces 
the  capacity  and  increases  the  cost  per  ton  of  operating  the  plant. 


Fig.  4. — Effect  of  intermittent  operation  on  dehydrating  time  of  walnuts. 
Drying  temperature,  110°  F. 

Some  operators  thought  that  during  the  nightly  shut-down  period 
a  considerable  amount  of  moisture  equalization  occurred  between  the 
already  dried  and  the  still  moist  nuts  which  might  be  mixed  together 
in  the  bins.  For  the  purpose  of  ascertaining  the  extent  of  such 
equalization,  a  lot  of  thoroughly  cured  nuts  were  individually  marked, 
mixed  and  sacked  with  an  equal  weight  of  uncured  nuts.  At  periodic 
intervals,  the  respective  lots  were  separated  and  weighed.  The  results 
showed  that  the  absorption  of  moisture  by  the  cured  nuts  from  the 


BULLETIN   376]  SUN-DRYING  AND  DEHYDRATION   OF   WALNUTS  17 

uncured  nuts  was  negligible  and  therefore  of  no  value  in  obtaining 
even  drying. 

In  other  words,  the  drying  of  each  individual  nut  proceeds  inde- 
pendently until  its  moisture  content  reaches  an  equilibrium  with  the 
moisture  content  of  the  surrounding  air. 

RELATION  OF  HUMIDITY  TO  DEHYDRATION 
The  lower  the  relative  humidity  of  air  the  greater  its  moisture 
absorbing  power.  The  relative  humidity  of  air  is  halved  and  conse- 
quently its  moisture  holding  capacity  doubled  for  every  27°  F.  rise 
in  temperature.  Hence,  outside  air  saturated  with  moisture  at  56°  F. 
would  have  its  moisture  holding  capacity  quadrupled  when  heated  to 
110°  F.  in  a  dehydrater.  This  explains  why  dehydraters  continue 
drying  irrespective  of  external  atmospheric  changes. 

No  specific  experiments  were  made  on  the  effect  of  air  of  different 
degrees  of  humidity  on  the  drying  time  of  walnuts,  but  it  was  observed 
incidentally  that  a  material  increase  in  the  relative  humidity  of  the 
air  resulted  in  a  slightly  retarded  rate  of  drying  of  the  nuts.  However, 
there  is  no  danger  from  this  source  since  the  volume  of  air  required 
to  convey  to  the  nuts  the  necessary  heat  for  moisture  evaporation  is 
several  times  that  required  to  absorb  this  evaporated  moisture.  Hence, 
the  retardation  of  drying  by  a  wholly  or  relatively  heavily  saturated 
air  is  rarely  encountered  in  walnut  dryers,  except  in  the  case  of  a 
natural  draft  dryer  operated  with  closed  vents. 

RELATION  OF  AIR  FLOW  TO  DEHYDRATION 
Air  currents  serve  two  essential  functions  in  any  dehydrater :  first, 
to  convey  the  heat  required  for  moisture  evaporation  from  the  source 
of  heat  to  the  material  to  be  dried,  and  second,  to  absorb  and  carry 
away  the  water  vapor  produced.  The  amount  of  evaporation  in  a 
given  time  is  dependent  on  the  amount  of  heat  reaching  the  nuts.  The 
amount  of  heat  is  in  turn  controlled  by : 

a.  The  temperature  of  the  air. 

b.  The  volume  of  the  air. 

It  has  already  been  shown  that  the  temperature  of  the  air  must 
not  exceed  110°  F.  Consequently,  any  increase  in  the  speed  of  drying 
must  be  brought  about  by  increasing  the  volume  of  air  and  therefore 
the  amount  of  heat  reaching  the  nuts  in  a  given  time. 

Air  flow  in  dehydraters  is  secured  in  one  of  two  ways: 

a.  Natural  draft  produced  by  the  expansion  of  heated  air. 

b.  Forced  draft  produced  by  power-driven  fans. 


18 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


The  latter  method  is  used  in  most  modern  dehydraters  because 
natural  drafts  are  usually  inadequate  in  volume  and  difficult  to 
properly  direct  and  distribute.  The  effect  of  increased  air  flow  on 
reducing  the  drying  time  of  walnuts  can  be  seen  in  table  4.  In  walnut 
tray  dehydraters,  velocities  in  excess  of  500  feet  per  minute  over  the 
trays  do  not  warrant  the  increased  expenditure  for  such  air  flow.  On 
the  other  hand,  air  velocities  much  below  500  feet  per  minute  greatly 
increase  the  time  and  cost  of  drying. 

TABLE  4 

Eelation  of  Air  Velocity  to  Drying  Time  of  Walnuts 


Method  of  drying 

Average 
temperature 

Air 

velocity 

(ft.  per  min.) 

Drying 
time 
(hours) 

Per  cent 

water 
lost 

Sun-dried 

72 
100 
100 
100 

250 

500 

1000 

122 
40 
32 

28 

27 

Dehydrater 

24 

Dehydrater 

25 

Dehydrater 

21 

Dehydrater 

100 
100 

Nat.  Draft 
500 

33 
16 

9 

Dehydrater 

9 

Most  walnut  dryers  built  so  far  are  imperfect  because  of  inadequate 
and  uneven  air  flow.  Only  by  the  use  of  fans  of  sufficient  power  can 
the  most  rapid  and  uniform  drying  be  obtained.  Dependence  on 
natural  draft  or  the  use  of  fans  of  insufficient  capacity  is  false  economy. 


EELATION  OF  DEPTH  OF  WALNUTS  TO  AIE  FLOW 
Since  the  bin  type  of  dryer  seems  to  be  the  most  generally  satisfac- 
tory for  walnuts,  an  experiment  was  made  to  determine  the  effect  of 
various  depths  of  walnuts  on  the  flow  of  air  through  them.  From  the. 
graphic  presentation  of  the  results  in  figure  5,  it  can  be  seen  that  the 
first  6  to  12  inches  of  walnuts  practically  determines  the  volume  of  air 
which  a  given  fan  can  force  through  the  nuts.  Increasing  the  depth 
of  nuts  does  not  alter  the  free  area  for  air  passage  but  merely  decreases 
the  air  flow  slowly  in  proportion  to  the  increased  frictional  resistance 
to  its  passage.  In  bin  dryers  the  heated  air  should  be  delivered  from 
the  fan  to  the  bottoms  or  sides  of  the  bins  by  the  largest,  shortest,  and 
straightest  flues  which  can  be  used.  The  air  should  enter  at  a  central 
point  from  which  it  can  be  equally  distributed  to  all  parts  of  the  bin. 
This  can  be  accomplished  by  causing  the  air  to  enter  at  the  center  of 
the  bottom  or  side  of  the  bin  from  where  it  can  be  evenly  distributed 
over  the  entire  bin  by  proper  arrangement  of  baffles. 


BULLETIN   376]  SUN-DRYING  AND   DEHYDRATION   OF   WALNUTS 


19 


2300 


Ifc      16*      Z*      30     36     H-Z     H8      5+     60 

Depth    of  Walnuts  (inches) 

Fig.  5. — Effect  of  depth  of  walnuts  on  air  velocity. 


20  UNIVERSITY    OF   CALIFORNIA — EXPERIMENT    STATION 


YIELDS  AND  STOEAGE  OP  DEHYDRATED  WALNUTS 

Since  the  curing  of  walnuts  consists  of  the  evaporation  of  excess 
moisture,  there  should  be  no  difference  in  yields  by  sun-drying  or 
dehydration,  provided  the  nuts  are  reduced  to  the  same  degree  of 
dryness  in  both  cases.  From  table  3  it  can  be  seen  that,  although  there 
were  occasional  differences  in  the  losses  in  weight  by  the  two  methods 
of  drying,  these  differences  were  not  consistently  in  favor  of  either 
method  and  that  the  seasonal  averages  were  almost  identical. 

Figure  6  shows  the  average  changes  in  weight  during  storage  for 
the  dehydrated  and  for  the  sun-dried  nuts  referred  to  in  table  3.  The 
unusually  dry  winter  of  1923-24  in  southern  California  caused  both 
lots  to  lose  over  2  per  cent  of  their  weight  in  storage,  but  during  a 
period  of  relatively  high  humidity  caused  by  spring  rains  the  nuts 
regained  their  original  weight,  showing  that  they  had  been  dried  to 
the  proper  degree  for  normal  winter  storage.  The  dehydrated  nuts 
had  evidently  been  cured  a  little  more  thoroughly,  as  shown  by  the 
average  moisture  contents  given  at  the  bottom  of  column  5,  table  3. 
This  was  confirmed  by  their  slightly  smaller  change  in  weight  during 
storage. 


SEPT  2  7       OCT.  15  NOV.  15  0€C.  //  JA/v.  /J 

DA  TE     WE'GHED 


Fig.  6. — Changes  in  weights  of  walnuts  during  storage  (Riverside,  1923-24). 


BULLETIN   376]  SUN-DRYING  AND  DEHYDRATION   OF   WALNUTS  21 


TYPES    OF    WALNUT    DEHYDRATERS 

It  is  not  the  purpose  of  this  publication  to  present  complete  plans 
and  specifications  for  dehydrater  construction.  However,  a  brief 
description  of  the  more  common  types  of  walnut  dehydraters,  with 
their  relative  advantages  and  disadvantages,  should  be  of  interest  to 
prospective  purchasers. 

NATURAL  DEAFT  DEYEES 

Probably  the  simplest  form  of  dryer  consists  in  closing  drying 
houses,  such  as  the  Sidwell  Dryer  described  on  page  7,  with  solid 
walls  and  placing  one  or  more  small  furnaces  on  the  ground  under  the 
bottom  floor.  This  permits  such  a  drying  house  to  continue  drying  in 
cold  or  damp  weather,  because  the  outer  air  drawn  in  through  openings 
near  the  ground  is  heated  by  passing  over  the  furnaces,  rises  through 
the  floors  of  nuts  and  eventually  escapes  through  a  ventilator  in  the 
roof,  carrying  with  it  some  moisture  evaporated  from  the  nuts. 

In  another  type  of  natural  draft  dryer  a  series  of  vertical  screen 
walls,  some  6  inches  apart,  is  substituted  for  the  screen  floors.  The 
space  between  each  alternate  pair  of  walls  is  filled  from  above  with 
nuts,  the  alternate  open  spaces  being  reserved  for  the  upward  passage 
of  heated  air. 

These  and  similar  modifications  of  natural  draft  dryers  have  two 
distinct  advantages  over  sun-drying  in  that  they  reduce  labor  in 
handling  the  nuts  and  continue  drying  in  all  kinds  of  weather.  From 
the  standpoint  of  efficient  dehydration,  however,  such  dryers  are 
imperfect.  They  occupy  more  space  than  bin  type  dehydraters  of 
equal  capacity.  The  air  flow  is  only  sufficient  for  relatively  slow 
drying.  Since  natural  air  currents  follow  paths  of  least  resistance,  it 
is  very  difficult  to  distribute  the  air  and  heat  so  as  to  insure  even  dry- 
ing, especially  when  the  dryer  is  only  partly  filled.  The  temperature 
in  natural  draft  dryers  is  subject  to  wide  fluctuations  which  cannot 
be  regulated  without  close  watching. 

AIE  BLAST  DEHYDEATEES 

The  use  of  fans  in  dehydraters  to  furnish  a  strong,  constant  flow 
of  heated  air,  the  temperature  and  distribution  of  which  are  under 
rather  exact  control,  has  now  become  almost  universal.  Although 
there  are  numerous  modifications  in  this  class,  the  tunnel  type  using 
trays  and  the  bin  type  are  of  greatest  interest  because  both  have  been 
successfully  used  on  walnuts  for  several  years. 


22 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


Bin  Dehydraters  are  a  class  developed  for  the  drying  of  walnuts 
only.  They  consist  of  three  essential  parts:  one  or  more  bins  for 
holding  the  nuts,  a  furnace  or  other  device  for  generating  heat,  and  a 
fan  for  conveying  heated  air  from  the  furnace  to  the  bins. 

One  type  consists  of  bins  about  10  feet  square,  the  bottoms  of  which 
are  made  of  slats  or  screen  and  in  the  form  of  inverted  pyramids.  In 
another  type  (illustrated  in  figs.  7,  8,  and  9)  rectangular  bins,  usually 
6  feet  by  16  feet,  are  built  on  about  a  30  degree  slope,  with  a  false, 
slatted  floor  forming  an  air  chamber  some  6  inches  deep  beneath  the 
nuts.  The  capacity  of  a  bin  dryer  may  be  determined  by  multiplying 
the  cubical  contents  by  35  pounds  of  cured  nuts  per  cubic  foot. 


Fig.  7. — Full  and  empty  bins  and  sacking  device.     Mahoney  bin  dryer. 


A  blast  of  heated  air  is  introduced  into  the  bottom  of  each  bin 
through  flues  leading  from  a  fan,  one  fan  and  furnace  usually  supply- 
ing a  battery  of  four  bins.  The  bins  are  filled  with  nuts  from  above  to 
a  depth  of  2  to  3  feet  by  means  of  a  simple  elevator  and  conveyor, 
giving  a  maximum  capacity  per  bin  of  2y2  tons  of  nuts. 

In  both  of  these  types  the  air  heating  equipment  is  located  close  by. 
This  usually  consists  of  either  a  standard  oil  burning  furnace,  such  as 
is  used  for  heating  buildings,  or  a  simpler  heating  chamber  utilizing 
direct  heat  from  electricity  or  gas.  Outside  air  is  drawn  through  the 
heating  chamber  by  a  fan  and  delivered  through  flues  to  the  bottoms 
of  the  several  bins.  It  then  rises  through  the  mass  of  nuts  and 
eventually  escapes  into  the  outer  air,  carrying  with  it  the  moisture 
which  the  heat  has  caused  to  evaporate  from  the  nuts. 


Bulletin  376] 


SUN-DRYING  AND   DEHYDRATION    OF   WALNUTS 


23 


The  best  type  of  fan  for  this  purpose  is  that  known  as  the  Multi- 
vane,  which  delivers  the  maximum  volume  of  air  for  the  horsepower 
expended  and  operates  successfully  against  considerable  back  pressure 
caused  by  resistance  to  the  flow  of  the  air  through  the  bins  of  nuts. 

The  time  required  in  such  dryers  varies  according  to  the  moisture 
content  of  the  nuts  and  the  operating  temperature.  Constant  operation 
at  100°  F.  to  110°  F.  will  complete  drying  in  24  to  48  hours.    Proper 


Fig. 


-Elevator  and  conveyor  for  filling  a  Mahoney  bin  dryer. 


construction  and  arrangement  of  the  bins  and  air  ducts  will  result  in 
uniform  distribution  of  heated  air  throughout  the  bin  and  consequently 
in  even  drying. 

When  the  nuts  are  dry  they  are  either  drawn  by  gravity  directly 
into  sacks  from  the  bottom  of  each  bin  or  discharged  into  a  common 
conveyor  which  carries  the  nuts  to  a  central  point  for  sacking. 


24 


UNIVERSITY    OF   CALIFORNIA — EXPERIMENT    STATION 


These  bin  dryers  are  generally  considered  the  most  satisfactory 
type  for  walnuts  yet  developed.  They  provide  the  maximum  daily 
capacity  for  the  investment  involved  and  the  space  occupied.  They 
are  susceptible  of  exact  yet  simple  regulation  of  the  temperature  and 
distribution  of  air  and  consequently  permit  the  most  rapid  and 
thorough  drying  consistent  with  quality.  They  are  most  economical 
of  labor,  one  man  being  easily  able  to  load,  dry,  unload  and  sack  5  tons 
of  nuts  daily. 

The  dryers  of  this  type  so  far  constructed  have  not  realized  their 
greatest  possible  efficiency  because  of  the  use  of  fans  and  motors  of 
inadequate  size.  The  lower  the  air  flow  the  slower  and  more  uneven 
the  drying.  It  is  also  probable  that  recirculation  of  the  warm  exhaust 
air,  as  practiced  in  all  modern  fruit  dehydraters,  would  result  in 
greater  fuel  efficiency. 


Fig.  9. — Furnace,  fan  and  flues  for  distributing  heated  air  to  a  four-bin 

Mahoney  dryer. 


Tray  Dehydraters. — Tray  dehydraters  for  fruits  are  now  in  exten- 
sive use  in  California  as  well  as  in  the  Pacific  Northwest.  The  soft 
texture  of  all  fruits  except  apples  necessitates  the  use  of  trays  in  dry- 
ing. There  are  many  types  of  tray  dehydraters  in  use,  but  the  type  con- 
sidered most  generally  satisfactory  is  known  as  the  Tunnel  dehydrater, 
consisting  of  a  long  chamber  6  or  more  feet  wide  by  7  feet  high.  The 
trays  are  entered  at  the  cooler  end  on  slides  or  cars  and  moved  pro- 
gressively toward  the  hotter  end  from  which  they  are  removed  when 


BULLETIN   376]  SUN-DRYING  AND  DEHYDRATION   OF   WALNUTS  25 

the  contents  are  dried.  Some  of  the  small  dehydraters  depend  on  a 
natural  draft  of  heated  air,  but  most  of  the  modern  installations 
include  a  furnace  and  fan  located  in  an  auxiliary  chamber  alongside, 
above  or  below  the  drying"  tunnel  in  such  a  way  that  a  powerful,  con- 
trolled stream  of  heated  air  is  blown  between  the  trays  of  material  in 
the  tunnel  and  either  discharged  or  recirculated  from  the  exhaust  end 
of  the  tunnel. 

This  type  of  dehydrater  has  been  successfully  used  on  walnuts  in 
both  Oregon  and  California.  It  gives  the  most  rapid  and  uniform 
drying  of  all  types  and  is  most  economical  of  fuel.  However,  the  first 
cost  is  greater  for  its  capacity  than  that  of  bin  dryers  and  the  extra 
labor  required  in  loading  and  unloading  the  trays  makes  the  cost  of 
drying  walnuts  considerably  greater.  The  capacity  of  a  tray  dehy- 
drater may  be  determined  by  multiplying*  the  total  square  feet  of  tray 
surface  by  2  pounds  of  cured  nuts  per  square  foot  for  trays  filled 
one  nut  deep.  Except  for  growers  who  desire  to  dry  other  fruits  as 
well,  tray  dehydraters  are  not  advisable.  Since  the  economic  dehy- 
dration of  all  fruits  necessitates  the  use  of  higher  temperatures  than 
are  permissible  for  walnuts,  fruits  and  walnuts  cannot  be  dried 
simultaneously. 


MECHANICAL  MOVEMENT  OF  NUTS   DURING   DRYING 

Dehydraters  involving  mechanical  devices  for  the  constant  or  inter- 
mittent movement  of  nuts  during  drying  are  unnecessarily  complicated 
and  expensive  to  build  and  operate.  It  is  sounder  in  principle  to  bring 
the  warm  air  to  the  nuts  than  to  attempt  to  bring  the  nuts  to  the  air. 


COSTS   OF   DEHYDRATION 

The  advantages  of  walnut  dehydration  listed  on  page  10  would  be 
of  little  interest  to  growers  if  such  advantages  could  only  be  obtained 
at  greatly  increased  costs.  The  use  of  walnut  dehydraters  has  not  yet 
become  sufficiently  general  to  permit  of  the  gathering  of  extensive  data 
on  the  cost  of  dehydration.  However,  the  costs  of  operation  presented 
in  table  5  are  considered  typical  of  present  practice. 

It  can  be  seen  that  although  the  fixed  charges  per  ton  for  the  bin 
dryer  are  greater  than  for  sun-drying,  because  of  the  greater  invest- 
ment, the  operating  costs  are  sufficiently  lower,  because  of  labor  saving, 
to  make  the  total  cost  about  the  same.  The  moderate  cost  of  power 
and  fuel  is  easily  counterbalanced  by  the  saving  in  labor.    It  is  reason- 


26 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


able  to  expect  that,  as  the  construction  and  operation  of  walnut  dehy- 
draters  is  perfected,  their  cost  of  operation  will  decrease  to  an  amount 
less  than  that  of  sun-drying,  as  already  realized  in  the  case  of  modern 
prune  dehydraters, 

TABLE  5 
Comparative  Costs  per  Ton  of  Sun-drying  and  Dehydrating  Walnuts  in  1923 


Sun  drying 

Bin  dryer 

Tray  Dryer 

Labor 

$3.77 

$1.61 
1.04 

.23 

$3    13* 

Fuel 

2.12f 

92 

Power 

Total  Operating  Cost 

$3.77 
1.71 

$5.48 

$2.88 
2.75 

$5.63 

$6  17 

Fixed  Charges! 

2  00 

Total  all  costs 

$8  17 

*  Includes  hulling  and  washing. 

t  Electric  heat;  equivalent  to  14  cents  if  oil  used. 

j  Interest  and  depreciation  at  11  per  cent. 


Walnut  dehydraters  are  still  in  a  state  of  development,  but  they 
have  already  sufficiently  established  their  desirability  to  warrant  care- 
ful consideration  by  all  walnut  growers.  Their  advantages  are 
obtainable  at  no  greater  final  cost  than  that  of  sun-drying.  Their  only 
disadvantage  is  the  greater  initial  cost  of  the  equipment. 

The  cost  of  a  completely  equipped  and  housed  walnut  dehydrater 
will  vary  considerably  with  the  type,  size,  materials  of  construction, 
etc.,  but  for  an  efficient  dehydrater  of  medium  size  will  approximate 
$500  to  $600  per  ton  daily  capacity. 


STATION  PUBLICATIONS  AVAILABLE  FOE  FREE  DISTRIBUTION 


BULLETINS 

No.  No. 

253.  Irrigation   and   Soil  Conditions  in  the  339. 
Sierra  Nevada  Foothills,  California. 

261.  Melaxuma    of    the    Walnut,    "Juglans  341. 

regia."  343. 

262.  Citrus   Diseases   of  Florida   and   Cuba  344. 

Compared  with  those  of  California. 

263.  Size  Grades  for  Ripe  Olives.  346. 
268.  Growing  and  Grafting  Olive  Seedlings.              347. 
273.  Preliminary  Report  on  Kearney  Vine- 
yard Experimental  Drain.                                  348. 

275.  The  Cultivation  of  Belladonna  in  Cali-  349. 

fornia. 

276.  The   Pomegranate.  350. 

277.  Sudan  Grass.  351. 

278.  Grain   Sorghums.  352. 

279.  Irrigation  of  Rice  in  California. 

280.  Irrigation  of  Alfalfa  in  the  Sacramento  353. 

Valley.  354. 

283.  The  Olive  Insects  of  California.  355. 

285.  The  Milk  Goat  in  California.  357. 

286.  Commercial   Fertilizers. 

287.  Vineerar  from  Waste  Fruits. 

294.   Bean    Culture   in    California.  358. 

298.   Seedless  Raisin  Grapes. 

304.  A  Study  of  the  Effects  of  Freezes  on  359. 

Citrus  in  California.  360. 

808.  I.  Fumigation  with  Liquid  Hydrocyanic 

Acid.  II.  Physical  and  Chemical  Prop-  361. 

erties  of  Liquid  Hydrocyanic  Acid. 
312.  Mariout  Barley.  362. 

317.   Selections  of  Stocks  in  Citrus   Propa-  363. 

gation. 
319.   Caprifigs  and  Caprification.  364. 

321.  Commercial  Production  of  Grape  Syrup. 

324.  Storage  of  Perishable  Fruit  at  Freezing  366. 

Temperatures. 

325.  Rice  Irrigation  Measurements  and  Ex-  367. 

periments     in     Sacramento     Valley, 
1914-1919.  368. 

328.   Prune  Growing  in  California. 

331.   Phylloxera-Resistant  Stocks.  369. 

334.  Preliminary  Volume  Tables  for  Second-  370. 

Growth  Redwood.  371. 

335.  Cocoanut   Meal    as    a   Feed   for   Dairy 

Cows  and  Other  Livestock. 

336.  The  Preparation   of  Nicotine  Dust   as 

an  Insecticide. 


The  Relative  Cost  of  Making  Logs  from 
Small    and   Large   Timber. 

Studies  on  Irrigation  of  Citrus  Groves. 

Cheese  Pests  and  Their  Control. 

Cold  Storage  as  an  Aid  to  the  Market- 
ing of  Plums. 

Almond  Pollination. 

The  Control  of  Red  Spiders  in  Decidu- 
ous Orchards. 

Pruning  Young  Olive  Trees. 

A    Study    of    Sidedraft    and    Tractor 
Hitches. 

Agriculture  in  Cut-over  Redwood  Lands. 

California  State  Dairy  Cow  Competition. 

Further  Experiments  in  Plum  Pollina- 
tion. 

Bovine  Infectious  Abortion. 

Results  of  Rice  Experiments  in   1922. 

The  Peach  Twig  Borer. 

A    Self-mixing    Dusting    Machine 
Applying      Dry      Insecticides 
Fungicides. 

Black    Measles,    Water    Berries, 
Related   Vine   Troubles. 

Fruit  Beverage  Investigations. 

Gum  Diseases  of  Citrus  Trees  in  Cali- 
fornia. 

Preliminary  Yield   Tables   for   Second 
Growth  Redwood. 

Dust  and  the  Tractor  Engine. 

The  Pruning  of  Citrus  Trees  in  Cali- 
fornia. 

Fungicidal    Dusts   for   the    Control   of 
Bunt. 

Turkish  Tobacco  Culture,  Curing  and 
Marketing. 

Methods  of  Harvesting  and  Irrigation 
in  Relation  to  Mouldy  Walnuts. 

Bacterial  Decomposition  of  Olives  dur- 
ing Pickling. 

Comparison  of  Woods  for  Butter  Boxes. 

Browning  of  Yellow  Newtown  Apples. 

The    Relative   Cost   of   Yarding    Small 
and  Large  Timber. 


for 
and 


and 


CIRCULARS 

No.  No. 
70.  Observations   on    the    Status   of    Corn  161. 
Growing  in  California.  164. 
87.  Alfalfa.  165. 
111.  The  Use  of  Lime  and  Gypsum  on  Cali- 
fornia Soils.  166. 
113.  Correspondence  Courses  in  Agriculture.  167. 
117.  The    Selection    and   Cost   of    a    Small  170. 

Pumping  Plant. 

127.  House  Fumigation.  172. 

129.  The  Control  of  Citrus  Insects.  173. 
136.  Melilotua   indica    as    a    Green-Manure 

Crop  for  California.  174. 

144.  Oidium  or  Powdery  Mildew  of  the  Vine.  175. 

151.  Feeding  and  Management  of  Hogs. 

152.  Some  Observations  on  the  Bulk  Hand-  178. 

ling  of  Grain  in   California.  179. 

153.  Announcement  of  the  California  State 

Dairy  Cow  Competition,  1916-18.  182. 

154.  Irrigation  Practice  in  Growing  Small 

Fruit  in  California.  184. 

155.  Bovine  Tuberculosis.  188. 

157.  Control  of  the  Pear  Scab.  190. 

158.  Home  and  Farm  Canning.  193. 
160.   Lettuce  Growing  in  California.  198. 


Potatoes  in  California. 

Small  Fruit  Culture  in  California. 

Fundamentals   of   Sugar   Beet   Culture 

under  California  Conditions. 
The  County  Farm  Bureau. 
Feeding  Stuffs  of  Minor  Importance. 
Fertilizing  California  Soils  for  the  1918 

Crop. 
Wheat  Culture. 
The    Construction    of    the    Wood-Hoop 

Silo. 
Farm   Drainage  Methods. 
Progress  Report  on  the  Marketing  and 

Distribution  of  Milk. 
The  Packing  of  Apples  in  California. 
Factors   of   Importance   in    Producing 

Milk  of  Low  Bacterial  Count. 
Extending  the  Area  of  Irrigated  Wheat 

in  California  for  1918. 
A  Flock  of  Sheep  on  the  Farm. 
Lambing  Sheds. 

Agriculture  Clubs  in  California. 
A  Study  of  Farm  Labor  in  California. 
Syrup  from  Sweet  Sorghum. 


CIRCULARS — Continued 


No. 

199. 
201. 
202. 

203. 
205. 
206. 
208. 

209. 
210. 
212. 
214. 

215. 
217. 

218. 

219. 

228. 
230. 

231. 
232. 

233. 
234. 

235. 

236. 


237. 


238. 
239. 


240. 
241. 


Onion  Growing  in  California. 

Helpful  Hints  to  Hog  Raisers. 

County  Organizations  for  Rural  Fire 
Control. 

Peat  as  a  Manure  Substitute. 

Blackleg. 

Jack  Cheese. 

Summary  of  the  Annual  Reports  of  the 
Farm  Advisors  of  California. 

The  Function  of  the  Farm  Bureau. 

Suggestions  to  the  Settler  in  California. 

Salvaging  Rain-Damaged  Prunes. 

Seed  Treatment  for  the  Prevention  of 
Cereal  Smuts. 

Feeding  Dairy  Cows  in  California. 

Methods  for  Marketing  Vegetables  in 
California. 

Advanced  Registry  Testing  of  Dairy 
Cows. 

The  Present  Status  of  Alkali. 

Vineyard  Irrigation  in   Arid  Climates. 

Testing  Milk,  Cream,  and  Skim  Milk 
for  Butterfat. 

The  Home  Vineyard. 

Harvesting  and  Handling  California 
Cherries  for  Eastern  Shipment. 

Artificial  Incubation. 

Winter  Injury  to  Young  "Walnut  Trees 
during  1921-22. 

Soil  Analysis  and  Soil  and  Plant  Inter- 
relations. 

The  Common  Hawks  and  Owls  of  Cali- 
fornia from  the  Standpoint  of  the 
Rancher. 

Directions  for  the  Tanning  and  Dress- 
ing of  Furs. 

The  Apricot  in  California. 

Harvesting  and  Handling  Apricots  and 
Plums  for  Eastern   Shipment. 

Harvesting  and  Handling  Pears  for 
Eastern    Shipment. 

Harvesting  and  Handling  Peaches  for 
Eastern   Shipment. 


No. 
243. 

244. 
245. 
247. 
248. 

249. 
250. 

251. 


252. 
253. 
254. 

255. 

256. 
257. 
258. 
259. 
260. 

261. 
262. 
263. 
264. 

265. 
266. 

267. 

268. 

269. 
270. 
271. 


Marmalade  Juice  and  Jelly  Juice  from 
Citrus  Fruits. 

Central  Wire  Bracing  for  Fruit  Trees. 

Vine  Pruning  Systems. 

Colonization  and  Rural  Development. 

Some  Common  Errors  in  Vine  Pruning 
and  Their  Remedies. 

Replacing  Missing  Vines. 

Measurement  of  Irrigation  Water  on 
the  Farm. 

Recommendations  Concerning  the  Com- 
mon Diseases  and  Parasites  of 
Poultry    in    California. 

Supports  for  Vines. 

Vineyard  Plans. 

The  Use  of  Artificial  Light  to  Increase 
Winter  Egg  Production. 

Leguminous  Plants  as  Organic  Fertil- 
izer in  California  Agriculture. 

The  Control  of  Wild  Morning  Glory. 

The  Small-Seeded  Horse  Bean. 

Thinning  Deciduous  Fruits. 

Pear  By-products. 

A  Selected  List  of  References  Relating 
to  Irrigation  in  California. 

Sewing  Grain  Sacks. 

Cabbage  Growing  in  California. 

Tomato  Production  in  California. 

Preliminary  Essentials  to  Bovine  Tuber- 
culosis Control. 

Plant  Disease  and  Pest  Control. 

Analyzing  the  Citrus  Orchard  by  Means 
of  Simple  Tree  Records. 

The  Tendency  of  Tractors  to  Rise  in 
Front:  Causes  and  Remedies. 

Inexpensive  Labor-saving  Poultry  Ap- 
pliances. 

An   Orchard  Brush  Burner. 

A  Farm  Septic  Tank. 

Brooding  Chicks  Artificially. 


15m-5,'24 


